For the purpose of evaluating physical characteristics of various types of workpieces, optical measurement systems are commonly employed. With continuing improvements to opto-electronic devices and semiconductor technology in general, such measuring systems become particularly attractive as they interface well with high speed signal processing systems, either in the form of particularly tailored hardware or suitably programmed data processors. One area where the need for accurate measurement systems is of paramount importance is the lumber processing industry, especially the sawmill, where the workpiece of interest constitutes a tree stem containing a variety of discontinuities along its surface, e.g. chips, branch stubs, burrs, knots, etc. With improvements in log processing algorithms, e.g. those designed to define the manner in which the log or stem should be cut for optimal monetary yield, which are implemented using on-site data processing equipment, it is preferred that the data defining the physical characteristics of the stem be both as accurate as possible and be available within as short a time as possible in order to optimize the stem processing efficiency of the sawmill.
Unfortunately, conventional optical schemes for measuring tree stem characteristics are cumbersome and subject to inaccuracies introduced into the measurement process by the very nature of the sawmill itself. For example, in the U.S. Pat. Nos. to Denton 3,806,253, Callani 3,724,958 and Hammar 4,192,613 there are described optical measurement apparatus wherein sets of optical sensor elements and light emitting elements are disposed on opposite sides of the log. The detectors may be comprised of photodiode arrays and the generation of an output by each sensor depends upon whether or not the log or stem intercepts the beam emitted by the emitting elements, so that the thickness of the stem may be represented by the number of light emitting elements whose output beams are not detected by an associated photodiode.
These types of systems suffer from a number of deficiencies, not the least of which is the fact that the environment of a sawmill is extremely noisy with effectively every component in the mill being subject to continuous strong physical vibrations. Because the measuring components move with respect to one another and the stem itself moves, the data obtained by the approaches described in the above cited patent literature is subjected to a significant degree of error. Moreover, the danger of damage to the opto-electronic components is substantial where the measuring optics are physically located adjacent to the stem handling apparatus.